This invention provides an outboard-motor automatic disengaging clutch system and method.
On outboards with a forward-neutral-reverse gearbox, the operator must change gear with the motor at idle. The gearboxes are not synchromesh, but are instead slightly clunky devices that either engage or disengage the appropriate drive sprocket, depending on whether it is to be forward or reverse. There is no smooth transition from “go” to “whoa.” It is, instead, either in gear or out of gear.
The power from the crankshaft of the engine is driven to the gearboxes gear via the top “pinion.” This pinion then delivers the rotational power to the two gears beneath and meshing with it. These gears rotate in opposite directions. The pinion and gears are rotating all the time the engine is running. A lever, which usually is placed on the left side of the outboard or on a remote control, operates the clutch, which is located between the gears. The clutch is on a spline so it can slide back and forth while still maintaining rotational power. The clutch is moved in the desired direction to one gear or the other. When the clutch is meshed in with one of the gears, because of its matching shape, it rotates the propeller, which drives the boat. To shift to the other gear, the clutch is moved to the opposite gear. When the clutch is in the middle position between the gears, the gearbox is in neutral.
The clutch is a metal device shaped to mesh with the identically-shaped “teeth” on the inside of each gear cog. The clutch slides back and forth on a metal spline, moved by a “U”-shaped metal fingers or prong that slides the clutch back and forth between the two gears by way of the groove around the center of the clutch. In neutral, the clutch does not touch either gear, as it is between the two. Thus, if a gear change is made with the motor running quickly, the transition from neutral to drive can be lumpy and noisy, and a rough gear change may even damage the gears. All gears and the clutch are immersed in gearbox oil to reduce friction.
Outboard motors nearly all use a simple dog clutch type mechanism for connecting the propeller to the engine. This mechanism is an old technology that takes up very little room, can transmit a vast amount of torque in relation to its size, and is relatively cheap and simple to make. These attributes make it suitable for marine applications, where the gear case must be kept as small as possible to reduce underwater drag. The clutch dog is splined to the propeller shaft and allowed to move fore and aft sufficiently to engage the teeth machined on the side faces of forward and reverse gears. Both gears spin freely on the prop shaft, and are rotating whenever the engine is running. The dog clutch is moved to engage the desired gear by a cross pin and shift rod inside the hollow prop shaft. Because the gears are always rotating when shifting occurs, the dog clutch is subject to some wear and tear forces.
Maximum dog clutch life comes from following some simple rules. First, shift at the lowest possible speed, engine at idle and boat stationary. A low engine idle speed is key to the system. Second, always shift into gear quickly. Slowly shifting allows the dog teeth to “rattle” across the driven gear teeth, and the user instead would want the teeth to contact only once for each shift, for minimum wear. Third, do not shift when the engine is switched off (unless someone is turning the prop shaft). If the dog clutch and gear teeth meet end on, the shift linkages can be bend, and then end up in gear jumping trouble because the bent linkages will lead to improper engagement of the dog clutch. Lastly, regularly check the shift system adjustments to ensure the dog clutch is moving to full engagement.
Worn cables or linkages that reduce dog clutch travel can result in gear jumping under load, which rapidly destroys the side faces of the dog clutch teeth, and additionally applies a “hammer blow” type forces to the rest of the transmission. (Because of this, gear jumping is usually described as feeling somewhere between a severe engine misfire and an invisible giant beating on the back of the boat with a very large hammer.) Most large motor dog clutches have some “locking angle” machined into the dog teeth. This angle helps to hold the dog engaged, when the engine torque forces are trying to separate them. Wear and tear reduces the locking angle and the area of the dog teeth in contact with the gear. When the teeth are worn sufficiently that the dog can no longer resist the torque forces on it, it will slide away from the gear and gear jumping will occur. Gear jumping can have a very destructive effect on the whole drive system, so must be addressed immediately. If it is caught very early and is only caused by bad adjustments/worn links, then it can be fixed easily. But if it has been occurring for some time, then gear and dog replacement is required to cure the problem. Gears and dog clutches are surface hardened, so the urge to grab the hand grinder and reface the dog teeth must be resisted. If it is refaced, it will only be a relatively temporary fix, as the softer surfaces exposed will simply wear much quicker than the originals. The locking angle on dog clutch teeth also increases the force required to shift out of gear, especially on fast boats that use big pitch propellers. Most large outboards therefore have a “Shift Interrupter” device that momentarily interrupts ignition as it is shifted to take the load off the gears for a fraction of second. It is typically a spring-loaded switch in the shift linkages on the engine that will ground an ignition circuit if the load on the shift cable or links exceeds and preset minimum.
A conventional outboard motor has a forward-neutral-reverse transmission in the lower unit, and is shifted into and out of forward and reverse from the neutral position. The forward-neutral-reverse transmission parts are in the lower unit. The lower unit is connected to the engine buy the mid-section housing. The engine's crankshaft is connected to the lower unit input gear by the input shaft. When the engine is running the input gear, forward output gear and reverse output gear in the lower unit are rotated around the output shaft by power from the engine through the input shaft.
The input shaft and gear, forward gear, and reverse gears are always rotating while the engine is running. However, even though they are rotating, they do not necessarily supply rotating power to the output shaft. The forward and reverse gears have an inside bore with a bearing that allow them to rotate around the output shaft while the output shaft remains still. The output shaft has a machined spline between the location of the input and output gears. This splined area is fitted with a mated clutch dog. On both end faces of the clutch dog are machined teeth. On the inner faces of both the input gear, and the output gear there are machined teeth that can mate with and are engaged by the clutch dog. The clutch dog is moved to engage with either the reverse gear or the forward gear by a shifting mechanism.
Different manufacturers have different designs of shifting mechanisms, but all move the clutch dog along the splined output shaft to engage with the selected gear.
When the engine is started, the clutch dog teeth are not engaged with either gear. After the engine is running, the operator of the vessel selects forward or reverse rotation of the propeller by moving the clutch dog to the selected motion by the shifting mechanism. At this time the forward and reverse gears are rotating with the engines power at approximately 400 to 500 rpm. The clutch dog, output shaft, and propeller are still. The clutch dog must mate with a gear and rotate the output shaft and propeller at an instant. At this instant the propeller shaft and propeller must go from still to the same RPM that the selected gear is rotating. Due to inertia and the water's resistance on the propeller's pitched blades, the engaging teeth of the gear and clutch dog sometimes deflect off of each other until engagement happens and is complete. Although the gears and clutch are precisely built and hardened, they do incur some wear during this shifting process. Additionally, if not shifted completely, damage may occur.
Most other non-marine forward-neutral-reverse gearboxes utilize a disengaging-engaging clutch when shifting the direction of travel. The driving component is disconnected from the gearbox input shaft. This disconnected area is fitted with a disengaging-engaging clutch that stops the rotation of the input shaft on the gearbox event though the engine is running. The stopped input shaft and gear allow shifting of the gearbox from forward to neutral to reverse while all gearbox components are not moving, also the input gear is not engaged to the engine or the output shaft at this time, so it is free to move into the correct place as the clutch dog moves out of and into the next selected gear.
Outboard motors are able to eliminate the disengaging-engaging clutch because the driven component is rotating in water. Accordingly, there is no positive engagement between the propeller and the water. There is, instead, what the marine world refers to as “slippage.” The initial load at start is much less than the load on a driven component on a ground traveling vehicle. A car at rest, with no disengage-engage clutch, cannot be put into forward or reverse with the engine running. The load on the wheels, ground, and driveline will not permit the gearbox to be engaged into forward or reverse while the input shaft and gear is rotating. The outboard motor on a boat at rest, since the load is much less between the propeller and water at the start of propeller rotation due to slippage, can be shifted into forward or reverse.
In the case of the outboard, if the engine is not idling slow enough, faster than the normal idle of the particular outboard motor, the transition into forward or reverse will be difficult. The forward and reverse gears are turning faster than they should be, and the linear motion of the clutch dog is not fast enough to engage the clutch dog teeth properly into the forward or reverse gear. The mating teeth deflect off of each other, causing premature wear or damage. This problem can also occur if the boat is resting in shallow water, or water with heavy vegetation. The propeller blades may be in contact with a muddy or hard bottom of the body of water, or grass or weeds that put heavier loading, due to less or no slippage on the output shaft and gears at propeller rotation start. This load on the propeller will cause extreme loading on the clutch dog and gear engaging system when transition from neutral into forward or reverse.
Even when the outboard motor is idling properly, and the propeller blades are in deep open water, the load at propeller rotation is not completely eliminated. The weight of the propeller, especially if stainless steel, and not aluminum, the resistance of the water to the propeller rotation, and the load of the water being pushed by the propeller as it starts to rotate, still cause somewhat rough and clunky shifting. This has always been accepted in the outboard motor world, but it is not ideal and there is room for improvement.
Lastly, when operating under conditions of mud or weeds, which strongly resist the rotation of the propeller, the present transmission systems of outboard motors are very unsatisfactory. When stuck in those conditions, the desired course of action is to run the propeller in reverse and with plenty of power. But, because the propeller is not at all free-spinning, shifting into reverse might be entirely impossible, and will be rough and clunky and damaging to the gear box even if the shift is made. There is therefore a need for a disengaging clutch system that will perform automatically and that can be retrofitted into existing outboard motors as well as being built into them.